Measuring and controlling system



2 Sheets-Sheet 1 Jan. 26, 1960 F. M. ALEXANDER MEASURING AND CONTROLLING SYSTEM Filed April 9, 195a I 3w v m uw 0mm M e omw o \..:t 2. omw m 7 awn/Wm g m M7 "N o v L Y J L r w J mw mm mm ,7 U h m m I I I 5 loo n63 6528 6528 I v om U m M 8 U (Ill Jan. 26, 1960 F. M. ALEXANDER 2,922,475

MEASURING AND CONTROLLING SYSTEM Filed April 9, 1958 DRb RRb i RESET BUTTON 2 Sheets-Sheet 2 DRo h FRo RRG BRIDGE PRe PM PM FRd FC PC 635. a 4% am h MEASURING AND CONTROLLING SYSTEM Frank M. Alexander, Columbus, Ohio, assignor to Industrial Nucleonics Corporation, a corporation of Ohio Application April 9, 1958, Serial No. 727,456

8 Claims. (Cl. 162-252) This invention in general relates to papermaking machinery and processes and more specifically ,to a measuring and controlling system for more accurately and effectively removing moisture from paper mat in a paper process.

The general method of papermaking is essentially the same for all types of machines. The web of paper is first formed from an aqueous suspension of fiber, on the traveling wire as in the case of the Fourdrinier machine, and then the mat of paper undergoes a dewatering trans formation. In Fourdrinier machines, the system of water removal involved can be classified into three component sections, the Fourdrinier or vacuum section, the presses and the dryer.

Each of these sections functions to remove as much moisture from the mat as the economics of their operation will permit. The sections can be adjusted through their own secondary loop controls to adhere somewhat to an optimum or target percentage of stock water removal. Thusly, a band or allowable percentage range of moisture removal can be programmed into the operation of each of the secondary loop controlss The removal of water by the vacutun system is the cheapest method until an upper limit is reached. At this point the rate of water removal becomes so great that the fibers in the paper mat are disturbed and begin to destroy the equality of the sheet. The press section is the second least expensive means of removing water between the two limits. The upper limit is that point at which the squeeze of the press rolls begins to damage the paper mat and/or the press felts. The lower limit is determined by that point at which the press rolls no longer contact the sheet evenly and, consequently, cause a moisture variation to occur across the web. The dryer section is the most expensive means of removing water. In this section, however, there is a lower limit defined by a minimum steam temperature and pressure such that if the lower limit is exceeded, condensation of the steam in the dryer will result, and there will be uneven drying across the paper sheet. The upper limit of control of the dryer is determined by that point where the heat is so great that it will cause excessive drying or even scorching of the surface of the paper sheet.

The co-ordinated control system of the present invention is devised to automatically scan the gauges indicating water removal in each of the three process systems and readjust their target or optimum moisture removal settings so as to produce through the control systems thereof the quality of paper desired in the most economical fashion. In this manner, the quality of the paper produced by the machine can be maintained consistent while the production process is operated at a minimum of expense.

Accordingly, it is an object of the present invention to provide in a paper making process method and means for accurately and efiectively removing moisture from the paper mat.

It is a further object of the present invention to autoatent O matically scan the three component sections of a paper process to determine in which of the three sections water may be more economically controlled.

It is another object of this invention to co-ordinate the control systems of each of the three hereinabove mentioned section control loops, in such a manner so as to effect the most economical overall stock de-watering process. I

It is still another object of the invention to provide a co-ordinated water control system that is relatively inexpensive in construction, and readily adaptable to paper-making machines in usetoday.

Further objects and attainments of the invention will become apparent from the following detailed description when taken in conjunction with the drawings in which:

Fig. 1 is a papermaking machine partly schematic and partly diagrammatic which incorporates the measuring and controlling system of the inventions;

Fig. 2 is a detailed schematic showing of the circuits shown in blocks 41 and 42 of Fig. 1, and

Fig. 3 is a detailed schematic showing of the circuits shown in block 40 of Fig. 1.-

Referring to Fig. 1, there is shown a conventional Fourdrinier paper process system incorporating the teachings of the present invention. Generally the three primary sections, Fourdrinier, presses and dryers are shown as sections A, B and C. In operation of'this system a controlled flow of stock input 1 is shown entering a headbox 2. Regulating this input is a flow valve 3 which, in essence, maintains the correct longitudinal profile of the paper sheet being produced. The regulating valve 3 is controlled from line 4 by the non-contacting basis weight or thickness gauge 5 at the dry end of the water removal process. Were it not for the controlled input to the headbox at 2, the secondary loop control processes 8, 21 and 31 would have to assume control of this longi-' tudinal weight or thickness variable, thus negating the effectiveness of an overall scanning type of control system.

The paper mat is initially formed on the Fourdrinier section, and it is herethat most of its moisture is most economically removed leaving a sheet which is approximately 88% moisture. Capillary action caused by table deflector rollers at 6 under the wire initiates water removal; however, immediate control at these rolls is not feasible. The suction boxes 7 following the rollers 6 provide a medium for automatic control of de-watering across the wire. This forced filtration on the wire, depicted at 7, is shown to be operated by a secondary loop control mechanism 8. Loop control 8 functions to adjust suction on the wire at 7 so as to maintain within pre-determined limits the percentage of moisture removed from the wire. These limits are programmed to correspond with the optimum operational characteristics of the particular Fourdrinier machine involved and the particular stock being run.

A voltage signal correlative to the amount of water removed on the wire section provides the basic operating signal for loop control 8 and a recording indicator showing water removed at 9. This control voltage is, in essence, the difference between a voltage correlative to pounds per minute or quantity of stock input, as shown on a gauge indicator at 10, and a voltage correlative to the quantity of stock present in the mat at the end of the suction boxes 7. As the stock input at the headbox 2 is held constant by the control described hereinabove, the difference voltage will represent the amount of water removed on the Fourdrinier section of the process. It is this difference voltage, representing water removed within a particular component section of the overall process, that is used as signal input to the process scanning control system to be described hereinafter.

Following the Fourdrinier section, the mat enters the press section where roll pressure from the rolls 20 removes moisture to result in a sheet which is approxi mately 67% moisture. Roll adjustment at 20 is controlled by a secondary loop control 21 which, as in the previous control section, operates from a voltage signal correlative to the amount of water removed by the process. This signal representing water removed by the presses is developed as the difference between the weight per unit area of mat material or basic weight measured at the non-contacting basic weight gauge 11 and the basic weight measured by the non-contacting gauge 22 following the press section. A water removed recorder 23 presents a record of this signal.

Having progressed through the presses, the mat 100 now enters the dryer section where moisture is removed by a steam heat exchange process shown as 30. Because of the expense of dryer operations, it is imperative that the mat 100 be at optimum moisture percentage before passing through these dryers. Control of heat input to the dryer over the range of moisture control is afiected by loop control 31. This control operates on a water removed signal wherein the signal is in eifect the difierence between the basic weight measurement at gauge 22 and the basic weight reading at gauge 5. Water removed at the dryer section is shown on a recording indicator 32.

' A moisture gauge 50 measures the amount of water remaining in the mat 100 after the water removal process. The Water in the mat is shown on a recording indicator 52. Combining the signal from the moisture gauge 50 with the signal from the basic weight gauge in a ratio computer 5 1 gives a signal indicative of the percent by weight of water remaining in the mat 100. This percentage is shown on indicating recorder 53. It is this final Water percentage signal that initiates action by the co-ordinated control system of the present invention to rebalance the water removal process.

In efiecting the most economical operation of the overall process of stock de-watering, a scanning control device is interconnected to each of the three secondary loop controls hereinabove outlined. Since the optimum operation of the dewatering process will result in the mat 100 leaving the dryer section 30 with a specific amount of water remaining in the mat for each specification or type of paper and since this amount of water is normally referred to as percent moisture by those skilled in the art of papermaking, a pre-determined percent moisture figure is fixed. Any deviation from this target at the indicator 53 initiates action by the overall scanning control. This action is efiected through the triggering device 40 which activates the scanning device 41. The scanning device 41 performs a comparison between existing and optimum water removed at each water removed indicator 9, 23 and 32.

The scanning device 41 causes the target reset device 42 to adjust one of the target settings at indicator 9, 23 or 32 according to conditions present at the time action was initiated. Since within certain limits water removal cost is highest in the dryer section 30 and lowest in the Fourdrinier section or wire 7, action indicated by the scanning device 41 depends upon whether the indicated moisture percentage at 53 is above or below the desired value. If the final moisture content is below the desired value,.indicating too much Water is being removed, most economical operation will result if the water removed at the dryer section is reduced, so the target reset device 42 will adjust the target of the dryer section so the con trol loop 31 will operate to remove less Water in the dryer section 30. However, if one of the indicators 9 or 23 are on a safe operation limit for maximum water removed, the target of that section would be reset so its individual control loop 8 or 21 could operate properly.

If the indicated moisture percentage 53 is above the desired value, indicating not enough water is being removed, the scanning unit 41 determines which indicator 9 or 23 is furthest from its optimum operating point and causes the target reset 42 to adjust the one that is furthest from target. If indicator 9 is on a limit, indicating the Fourdrinier section is removing as much water as is economical for the particular stock making up the mat 100, the scanning unit causes the adjustment to be made at the press section by resetting the target at indicator 23. If indicator 23 is also on a limit, showing no more water should be removed at that point in the process, the scanning unit 41 causes the targetreset 42 to adjust the target at indicator 32 so more water will be removed in the dryer section. If all three indicators 9, 23 and 32 are on safe operating limits, a warning signal is energized to alert the machine operator that an unusual condition has occurred and some action is required by him.

The secondary loop controls 8, 21 and 31 are proportional control systems of a type commonly known to those experienced in the automatic control field. Each controller receives an error voltage signal the amplitude of which is proportional to the difference between the position of the target and the chart trace representing water removed at the respective sections. The controller detects the phase and amplitude of this error signal and causes remedial action of proper direction and amount. The reset type of controller disclosed in US. Serial No. 688,720 for Electrolytic Plating Apparatus filed by Donald E. Varner on October 17, 1957 could be adapted for use in the system. i

The supervisory control contains three major sections, the trigger 40, scanning unit 41 and target reset 42. A typical and preferred arrangement of the trigger section is shown schematically by Fig. 2 and the scanning unit 2 by Fig. 3.

With reference to Fig. 2 the trigger unit or section is composed of a resistance bridge 60, a polarized relay 62, two output relays L and H, and associated circuitry arranged to provide for the operation of relay L if moisture percentage is less than the target and for the operation of relay H if moisture percentage is greater than the target. The resistance bridge 60 is made up of a potentiometer 68 whose movable arm 68a is set by the target set knob 70, a potentiometer 72 whose movable arm 72a is mechanically connected to the moisture percentage trace pen 74, a source of bridge voltage 76 and a series resistor 78 to limit current flow. The output of this bridge is fed through a current limiting resistor 80 to the polarized relay 62. This relay has characteristics such that when no current flows therethrough (when bridge is balanced), neither contact 62a nor 62b is closed to arm 620, a current flow through the relay 62 in one direction will cause one contact to be closed and a current flow in the other direction closes the other contact. The contacts 62a and 62b remain closed only during current flow. ,Thus, relay H is energized through one contact of the polarized relay 62 when the diiference between the position of the target potentiometer arm 68a and the percentage of water potentiometer arm 72a indicates the percentage of moisture is too high. Conversely, if the relative positions of the arms 68a and 72a of the potentiometers 68 and 70 respectively indicate the percentage moisture is too low, relay L will be energized through the other contact of the polarized relay 62.

Referring now to Fig. 3, the scanning unit 41 is composed of a motor driven multi-contact rotary switch 106, a motor 1%, several relays, a polarized relay 63, a difference amplifier 102 and their associated circuitry as shown. The scanning unit is to determine which section (Fourdrinier, press or dryer) is to be adjusted and to provide switching for the target reset unit to adjust the proper target setting to correct the moisture unbalance in the most economical way.

In the schematic representation of Fig. 3, contacts are identified by a letter or letters corresponding to the identification letter or letters given the relay or switch containing these contacts. The subscripts indicate dif ferent contacts on the same switch or relay. F, P and D are identifications of limit switches on the actuating means (valve, rheostat, etc.) of the control loops of the Fourdrinier, press and dryer sections, respectively. Actuation of one of these switches indicates that section is operating at the edge of its safe operating region and its control loop can make no further adjustments until conditions are changed. The relays DR, FR, PR, RC, PC, and RR are common type relays whose identification and functions will be explained in the following discussion of typical operation of the scanning unit. The difference amplifier 102 used in the comparison section is a device widely used and well known to anyone versed in the art of controls, computers or other electronic circuitry applications. The polarized relay 63 is similar to relay 62 described in the previous section concerning the trigger section. The resistance bridges 90, 92 and 94 are similar to the resistance bridge described with reference to Fig. 2 except that these bridges respond to variations in water removed in the Fourdrinier, press and dryer sections, respectively.

Operation of the scanning unit is initiated by closure of contact La or Ha as a result of operation of the trigger unit 40. Closure of La will cause the motor 108 to rotate the arm 106a of the rotary switch 106. This action is taken to determine if a limit switch has been actuated by one of the individual control limits. Since this condition would mean the percentage moisture was low and less water could be removed by some section, it is desirable that the section adjusted be the dryer section unless one of the other sections had hit a limit. As the motor rotates the switch arm across the first section, the relay DR will be energized unless a limit contact Fa, Pa or Da is open. If no limit contact is open and DR is energized, the motor rotation is stopped by opening of contact DRa. This maintains the switch contact to keep Dr energized until later in the operation. This relay then connects the output of the error bridge 94 connected with moisture removal indicator for the dryer section to the input of the target reset unit and initiates action by it. At the same time, DR connects the output of the target reset unit to the target adjusting motor for the Fourdrinier section water removal control loop.

The control action of the target reset unit can now take eflect. After the control is finished, and the adjustment made, a contact TRU from the target reset unit closes to energize the reset relay RR. The reset relay RR disconnects the motor lead 108a containing the contact Ha and connects through RRa the other lead 108!) of the motor. This causes the motor 108 to drive in one direction until the arm 1061b of the rotary switch 106 returns to the middle or start position. At this position the reset relay RR drops out stopping the motor 108. The reset relay RR, when it switches the motor 108, also disconnects the power to the trigger relays L and H by contact RRc and removes power from the arm of the rotary switch through contact RRb, thus dropping relay DR, PR or PR. When the target reset unit contact TRU opens, reset relay RR is de-energized and the trigger and scanning units are returned to normal position ready for another cycle.

If either the press or Fourdrinier section secondary control loop actuators are on a limit, the rotary switch 106 will be driven past the first segment since limit switch contact Fa or Pa would be open, preventing the energization of DR. If the Fourdrinier section limit is operated, the relay FR will be energized when the moving arm contacts its segment; this will stop the motor, switch the Fourdrinier water removed target bridge to the input of the target reset unit and connect the output of the target reset unit to the adjusting motor for the Fourdrinier section secondary control loop target. The

remainder of the cycle is the same as described previously. If the Fourdrinier section were not on a limit, the'arm 106a of the rotary switch 106 would be driven past the segment for that section to the segment for the press section. If this section control loop is on its limit, the sequence'is similar to above except relay PR is energized and does the switching.

In the event the dryer section limit is operated, the moving arm 106a: of the rotary switch 106 is driven on past the press section segment to the last segment on that side where relay FL is energized. This FL relay connects both the Fourdrinier and the press sections water removed bridges to the comparison circuit section through contacts FLa and FLb. The outputs of these bridges are then fed into the dilferential amplifier 102 such that the amplifier output is positive if the error is greater from one bridge and negative if the error is greater from the other bridge. The polarized relay CPR responds accordingly to energize either PC or PC and connect the proper bridge output to the target reset unit and connect the proper target adjusting motor to the output of the target reset unit. Thus, the target reset unit adjusts the target of the section Whose Water removal trace is furthest from optimum. The remainder of the cycle is similar to the other cases previously described.

The situation where the moisture percentage in the sheet is too high, requiring more water to be removed, is somewhat different than the cases just discussed. In this case, the trigger circuit will cause the motor to drive the switch arm to the side marked H where it first contacts the segment for the Fourdrinier section. This is accomplished by closure of contact Ha If the secondary control loop for the Fourdrinier section is operating norm-ally and the limit for this section is not operated, relay FL will be energized, switching the Fourdrinier and press section bridge output to the comparison section and the cycle proceeds as described in the previous paragraph.

If the Fourdrinier control loop limit is operated, the arm 106a of the rotary switch 106 moves on to the next segment energizing relay PR which connects the output of the press section water removed bridge to the target reset unit 42. This is done directly since there is no need for comparison if the Fourdrinier section can not be adjusted. The cycle then proceeds as before. In the event both the Fourdrinier and press section limits are actuated, the arm of the rotary switch moves to the segment for the dryer section, energizing relay DR and the cycle continues as before.

If all sections secondary control loops are inoperative due to limits being actuated the motor will drive the arm of the rotary switch past each of the segments of the Fourdrinier, press, and dryer sections to the segment for the warning device. This energizes relay W, stopping the motor and energizing 'a horn, light, hell or other device to alert the operator that an unusual situation exists so he must push the reset button to return the system to normal.

The target reset unit is a proportional controller arranged for discontinuous control. Once a target has been adjusted, no more action will be taken until enough time has elapsed for the section elfected to control to the new target and the effected material to reach the moisture gauge. This time is often called transportation lag. It is during this time that the contact TRU closes to energize the reset relay RR to reset the scanning and trigger units and return them to normal.

Since different sections of the paper maohine will have different transportation lags and since the least transportation lag possible is most desirable, contacts from relays FL, DR, FR and PR could be utilized to change the transportation lag time of the target reset unit to match the section adjusted.

It is apparent from the foregoing description that the present invention provides a means of operating the various water removal sections as close to an economic optimum as possible and, when an adjustment in optimum target is necessary, to make the adjustment to that section most economically adaptable to the change. Other arrangements may be had without departing from the true spirit and scope of the invention.

What is claimed is: I i

1. A paper making machine having at least a first, second and third water removal section, a basis weight gauge positioned within each of'said sections, a loop controller connecting each of said gauges to the water removal means in each of said sections, said controllers including a voltage generator operative to develop a voltage cor relative to the amount of water removedfrom its respective section, and to control the amount of water to be removed from each of said sections in accordance with a preset amount; a moisture gauge positioned adjacent the paper after it has been processed by said sections and operative to develop a voltage indicative of the amount of water in said processed paper, means for combining said last named moisture voltage and said voltage from the preceding loop controller to develop a voltage proportional to the percentage of moisture in said processed paper; a scanning device interconnected to each of said'loop controllers and to said combining means, and means controlled by said scanning device for resetting said preset amount of water-to be removed in each of said sections when said percentage voltage deviates from said preset amount.

2. A paper making machine having at least a first, second and third water removal section, a basis weight gauge positioned within each of said sections, a loop controller connecting each of said gauges to the water removal means in each of said sections, said controllers including a voltage generator operative to develop a voltage correlative to the amount :of water removed from its respective section and to control the amount of water to be removed from each of said sections in accordance with a preset amount; a moisture gauge positioned adjacent the paper after it has been processed by said sections and operative to develop a voltage indicative of the amount of water in said processed paper, means for combining said last named moisture voltage and said voltage from the. preceding loop controller to develop a voltage proportional to the percentageof moisture in said processed paper; a scanning device interconnected to each of said loop controllers and to said combining means, said scanning device operative to compare said voltage correlative to the amount of water in each section to said preset amount when said percentage voltageof said processed paper deviates from a predetermined amount, and means controlled by said scanning device for resetting said preset amount of water to be removed from a selected one of said sections when said percentage deviates above said predetermined amount and for resetting said preset amount of water to be removed from another selected one of said sections when said percentage voltage deviates below said predetermined amount.

3. A machine for-making paper from paper stock having a plurality of controllable water removal sections, each section operative to remove a certain percentage of water from said paper stock, means for determining the water content of the paper at each of said plurality of sections, means connected to said determining means for indicating the'percentage of water removed at each of said sections, means for establishing within upper and lower limits a preset amount of water to be removed from each of said sections, said preset amount closely approximating the optimum percentage of the water to be removed from each section, control means connected to each section and said preset means to control the amount of water removed in each section in accordance with said preset amount; a moisture gauge for measuring the amount of water in the paper after it has been procpaper deviates from a predetermined amount, and means,

connected to said presetting means and controlled by said scanning means for resetting said preset amount of water to be removed from each section when said preset amount has departed from said optimum percentages.

4. A machine for making paper from paper stock having a plurality of controllable Water removal sections, each section operative to remove a certain percentage of water from said paper stock, means for determining the water content of the paper at each of said plurality of sections, means connected to said determining means for indicating the percentage of water removed at each of said sections, means for establishing within upper and lower limits a preset amount of water to be removed from each of said sections, said preset amount closely approximating the optimum percentage of water to be removed from each section, control means connected to each section and said preset means to control the amount of Water removed in each section in accordance with said preset amount; a moisture gauge for measuring the amount of water in the paper after it has been processed by said sections, scanning means connected to each of said percentage of water indicators and including means for comparing the actual amount of water removed with said optimum amount for each section, means connected to said moisture gauge for actuating said scanning means when said moisture in said processed paper deviates from a predetermined amount, and means controlled by said scanning device for resetting said preset amount of water to be removed from a selected one of said sections when said percentage deviates above said predetermined amount and for resetting said preset amount of water to be removed from another selected one of said sections when said percentage voltage deviates below said predetermined amount.

5. A machine for making paper from paper stock hav-. ing at least a first, second and third water removal section, each section operative to remove a certain percentage of water from said paper stock, a non-contacting basis weight gauge positioned within each of said sections for determining the water content thereat, means for presetting within upper and lower limits an optimum amount of water to be removed from each of said sections, a loop controller connecting each of said gauges to the water removal means in each of said sections and said preset means, said controllers including a voltage generator operative to develop a voltage correlative to the amount of water removed from its respective section, indicating means connected to each of said voltage generators operative to convert said voltage to a percentage, a non-contacting moisture gauge positioned adjacent to the paper after it has been processed by said sections for developing a voltage'indicative of the amount of water remaining in said processed paper, means for combining said last named moisture voltage and said voltage from the preceding loop controller to develop a voltage proportional to the percentage of moisture remaining in said processed paper, a scanning device interconnected to said percentage indicator in each of said sections, said scanning means including means for comparing the actual and optimum amount of water removed for each section, means connecting said scanning means and said combining means for actuating said scanning means when said percentage voltage indicative of the water in said processed paper has deviated from a predetermined amount, and means controlled by said scanning means for resetting said amount of water to be removed from each section when said preset amount has departed from its optimum.

anaemia 6. A machine for making paper from paper stock having at least a first, second and third Water removal section, each section operative to remove a certain percentage of water from said paper stock, a non-contacting basis weight gauge positioned within each of said sections for determining the water content thereat, means for presetting Within upper and lower limits an optimum amount of water to be removed from each of said sections, a loop controller connecting each of said gauges to the water removal means in each of said sections and said preset means, said controllers including a voltage generator operative to develop a voltage correlative to the amount of Water removed from its respective section, indicating means connected to each of said voltage generators operative to convert said voltage to a percentage, a noncontacting moisture gauge positioned adjacent to the paper after it has been processed by said sections for developing a voltage indicative of the amount of water remaining in said processed paper, means for combining said last named moisture voltage and said voltage from the preceding loop controller to develop a voltage proportional to the percentage of moisture remaining in said processed paper, a scanning device interconnected to said percentage indicator in each of said sections, said scanning means including means for comparing the actual and optimum amount of water removed for each section, means connecting said scanning means and said combining means for actuating said scanning means when said percentage voltage indicative of the Water in said processed paper has deviated from a predetermined amount, and means controlled by said scanning means for resetting said amount of water to be removed from a selected one of said sections when said percentage deviates above said predetermined amount and for resetting said preset amount of water to be removed from another selected one of said sections when said percentage voltage deviates below said predetermined amount.

7. A paper making machine substantially as set forth in claim 1 wherein said voltages correlative to the amount of Water removed at said sections is the difierence between the quantity of stock input and stock present at each of said sections.

8. A paper making machine having at least Fourdrinier, press, and dryer water removal sections, a basis Weight gauge positioned within each of said sections, a loop controller connecting each of said gauges to the water removal means in each of said sections, said controllers including a voltage generator operative to develop a voltage correlative to the amount of water removed from its respective section, and to control the amount of water to be removed from each of said sections in accordance with a preset amount; a moisture gauge positiened adjacent the paper after it has been processed by said sections and operative to develop a voltage indicative of the amount of water in said processed paper, means for combining said last named moisture voltage and said voltage from the preceding loop controller to develop a voltage proportional to the percentage of moisture in said processed paper; a scanning device interconnected to each of said loop controllers and to said combining means, and means controlled by said scanning device for resetting said preset amount of water to be removed in each of said sections when said percentage voltage deviates from said preset amount.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,832 Krogh June 11, 1946 2,479,031 Tait Aug. 16, 1949 2,484,594 Spangenberg Oct. 11, 1949 

